In the crude oil market, there is a premium for refineries to process feeds derived from more refractory crudes into high value finished products. These refractory feeds are characterized by a higher density resulting from an increased fraction of aromatic compounds with a low hydrogen content. Therefore, these feeds require deeper hydrogenation (more extensive saturation of aromatic compounds) to produce fuels that meet specifications, such as viscosity, cold flow properties, cetane index, smoke point, emission requirements. The volumetric productivity of such a process provides higher benefits to refineries that deploy hydrogenation (HYD) catalysts powerful enough to meet the required hydrogenation activity.
Although hydrogenation is an important aspect of refining high density feeds, deep hydrogenation catalysts need to deliver more than maximizing the increase in volume between feed and refined products. In the wake of European environmental legislation, there is a global trend towards more stringent legislation that mandates environmentally friendly transportation fuels. An example is the mandate that all diesel produced in and imported into the US will have to be ultra-low-sulfur diesel (ULSD) as of Dec. 1, 2014. This change to transportation fuels with the low sulfur content allows for application of newer emission control technologies that should substantially lower emissions of particulate matter from diesel engines. Environmental mandates to remove contaminants like sulfur from refined oil products implies a need for catalysts which combine a deep hydrogenation (HYD) capability with a deep hydrodesulfurization (HDS) activity. This deep hydrodesulfurization activity is particularly important in refining high-density feed stocks, because a higher density typically implies a higher concentration of contaminants such as organic sulfur and nitrogen molecules, as well as metals and asphaltenes (i.e. multi-ring based aromatic compounds with a low solubility even in the most aromatic solvents).
Deep hydrodesulfurization is accomplished most efficiently through a combination of: 1) hydrogenation (HYD), which releases sulfur atoms after saturating the ring structure of parent aromatic compounds and 2) hydrogenolysis (HYL), which breaks the bond between a sulfur atom and the carbon atom(s) in the sulfur containing molecule, such as aromatic ring compounds. This implies that optimum catalysts for the deep hydrogenation of high-density feed stocks are expected to exhibit an appropriate balance between hydrogenation and hydrogenolysis functions. Catalysts that are suitable for hydroprocessing (e.g. hydrodesulfurization and hydrodenitrogenation) generally comprise molybdenum or tungsten sulfide or sulfocarbide, in combination with an element such as cobalt, nickel, iron, or a combination of thereof. Attempts have been made to modify the morphology of hydroprocessing catalysts to provide ways to control their activity and selectivity. For example, U.S. Pat. No. 4,528,089 discloses that catalysts prepared from carbon-containing catalyst precursors are more active than catalysts prepared from sulfide precursors without organic groups.
Considerable experimental and modeling efforts have been underway to better understand complex metal sulfide catalysts, including factors that control metal sulfide morphology. U.S. Pat. Nos. 7,591,942 and 7,544,632 demonstrate that sulfiding a self-supported multi-metallic catalyst in the presence of a surfactant amine gives a catalyst comprising stacked layers of molybdenum or tungsten sulfide with a reduced number of layers in stacks. A lower number of layers in stacks implies the presence of smaller crystals of molybdenum, tungsten or molybdenum tungsten sulfides, which can result in larger surface areas available for catalysis.
The saturation of aromatic compounds in distillate fractions, e.g. vacuum gas oil, heavy coker gas oil or diesel fuel has also drawn attention of researchers. A high aromatic content is associated with high density, poor fuel quality, low cetane numbers of diesel fuel and low smoke point values of jet fuel. High hydrodenitrogenation (HDN) activity is typically associated with aromatic saturation activity of nickel tungsten sulfides catalysts; while high hydrodesulfurization (HDS) activity associates with high hydrogenolysis activity of cobalt molybdenum sulfide catalysts.
There is still a need for improved catalysts with improved catalytic activity and resistance towards deactivation, specifically self-supported mixed metal sulfide catalysts for use in the hydroprocessing of lower grade, more refractory hydrocarbon feeds, capable of generating low aromatic products meeting new emission requirements. There is also a need for a better understanding of structure—performance relationships for these catalysts to design next generation of highly active self-supported catalysts for use in hydroprocessing.